The present invention relates to a process for decaffeinating raw coffee in which the caffeine is selectively separated from the raw coffee with the aid of a liquid solvent, i.e., a solvent which is liquid under normal conditions (1 bar and 20.degree. C.), which is saturated or even supersaturated with carbon dioxide at a temperature of 20.degree. C. to 110.degree. C. and a pressure of 30 bar to 300 bar so that a low caffeine or caffeine-free raw coffee is obtained which meets these definitions. On the other hand, the process according to the invention yields pure crystalline caffeine as a byproduct which can be utilized in pharmacology and in the beverage industry.
Many people frequently do not tolerate coffee because of its caffeine content. Many processes have therefore been developed for the purposes of extracting the caffeine from the raw coffee and simultaneously avoiding the removal of other raw coffee components which are necessary for the development of the aroma of the coffee during roasting. The caffeine is extracted from the raw coffee because if roasted coffee is decaffeinated a loss of aroma cannot be avoided.
In one prior art decaffeination process, the raw coffee is pre-treated, for example, in that the coffee beans are hydrolyzed by means of water vapor at an increased temperature, the coffee beans are extracted in a liquid-liquid extraction process by means of solvents, e.g. methylene chloride or ethyl acetate, then the solvent is removed from the raw coffee by evaporation and thereafter the moist raw coffee is dried. In this prior art process, some solvent residues may remain in the coffee and the raw coffee may be denatured to a certain extent.
Processes have also been proposed which use other solvents that need not be removed from the decaffeinated raw coffee. Of practical significance are primarily those processes which use water, supercritical carbon dioxide, liquid carbon dioxide or higher organic fatty acids (coffee oil) originating from coffee as solvents. These processes require long periods of extraction to remove caffeine to a sufficient degree. The reason for this is that the caffeine must first diffuse to the surface of the coffee beans in order to be taken up by the solvent. This process becomes slower and slower with decreasing caffeine concentration in the coffee beans. In many of these prior art processes, the solvent is circulated during the extraction process, thus causing high investment and energy costs.
DE-OS 3713953A1 which corresponds to U.S. Pat. No. 5,089,280 issued Feb. 18, 1992, discloses a process for decaffeinating raw coffee which essentially avoids the stated drawbacks. In this process, the raw coffee is moistened to a high water content of 35 to 50 weight percent and is then kept for a few minutes to several hours at a temperature of 20.degree. to 80.degree. C. in a gas atmosphere of 75 to 300 bar and stirred, if required. Thereafter, the pressure is reduced suddenly or within a few minutes, while avoiding freezing, to between 1 bar and p.sub.c (p.sub.c =critical pressure of the gas employed). To obtain a selective extraction of the caffeine, the raw coffee is rinsed with water or with the supercritical gas and the process is repeated several times if necessary. Thereafter, the raw coffee is pre-dried in a centrifuge and roasted. The caffeine is recovered from the rinsing agent in a known manner. This process requires a complicated and cost intensive repeated build-up of the pressure atmosphere.